A composite bridge amplifier is disclosed in U.S. Pat. No. 5,075,634. The amplifier comprises the series combination of a single-ended Class D amplifier and a linear Class AB bridge amplifier. The output of the Class D amplifier provides the supply voltage for the bridge amplifier. The signal to be amplified, e.g. an audio signal, varies the supply voltage to the bridge amplifier which produces a small and constant voltage drop across the output transistors in the bridge amplifier. This results in greatly reduced power dissipation in the output transistors, while providing Class AB type performance.
A feature of the amplifier disclosed in U.S. Pat. No. 5,075,634 is that the combination of bridge amplifier and Class D amplifier only requires a single-ended Class D amplifier. It will be appreciated that this can greatly reduce the cost of the amplifier. The Class D amplifier includes a pulse width modulator stage which controls the supply voltage to the bridge amplifier. The pulse width modulator (or pwm) stage comprises a pulse width modulator, a switch and a pwm filter. The power supply is coupled to the input of the pwm filter through-the switch which is controlled by the pulse width modulator. In the prior art amplifier, the control of the pulse width modulator comprises an open-loop system. A reference signal corresponding to the absolute value or full-wave rectified input audio signal is used to directly control or establish the pulse width for the pulse width modulator. In order to align the output of the pulse width modulator with the bridge audio signal, a time delay is needed.
The purpose of the time delay is to align or synchronize the output from the pulse width modulator stage with the bridge amplifier. There is lag between the supply voltage signal to the bridge and the audio signal input to the bridge due primarily to the reactance in the pwm filter. In the prior art amplifier, the supply signal and input signal were aligned by a time delay placed in the bridge audio signal path. Because the time delay is in the audio signal path, it must comprise a high quality circuit in order to maintain the high performance specifications for the amplifier. In practice, this usually means a digital implementation in order to provide high bandwidth and low distortion, which can add considerable expense to the cost of the amplifier.
The open loop control of the pulse width modulator stage in the prior art amplifier can affect the performance of the amplifier in other ways. Variations in the pwm filter, for example, due to the finite resistance of the switch and the reactive characteristics of the components in the filter, can cause error and distortion in the output from the pulse width modulator stage. Because the Class D amplifier provides the supply voltage for the bridge amplifier, distortion or error in the output from the pulse width modulator stage can lead to distortion and error in the bridge amplifier.
Another problem associated with the open-loop implementation for the pulse width modulator is the susceptibility to noise in the power supply. Since the power supply is connected (through the switch) to the pwm filter, any audio frequency noise on the supply will be coupled to the filter.
While the composite bridge amplifier disclosed in U.S. Pat. No. 5,075,634 provides a novel amplifier in which high efficiency can be achieved with less cost than known amplifiers, there is still a need for an improved composite bridge amplifier.